Conventionally a still image is created by capturing a single scene from video shot with a digital video camera or the like so that the still image has higher resolution than the frame images. Such a still image is created through a synthesis process that involves overlapping several frames contained in the video image.
For example, JP2000-244851A discloses a technique wherein a single frame image is selected as a reference image from among a number (n+1) of successive images, motion vectors for the other n frame images (processing-object-images) with respect to this reference image are calculated. and on the basis of the motion vectors, the (n+1) frame images are synthesized to produce a still image.
JP06-350974A discloses another technique for created a still image from interlacing video images, wherein one field is selected as a reference image from among a pair of interlacing fields, with the other filed being selected as a processing-object-image, then determining on a field-by-field basis whether the processing-object-image is suitable for synthesis, synthesizing them where determined to be appropriate. Typically, by increasing the number of frame images to be synthesized, it is possible to improve picture quality of the still image; however, it is not always the case that picture quality is improved by increasing the number of frame images to be synthesized. The reasons for this are described hereinbelow.
FIG. 1 is an illustration of a method for synthesizing a reference image and an object image for synthesis. At top in FIG. 1 are shown a reference image and an object image for synthesis, positioned so as to compensate for image shift. At bottom in FIG. 1 are shown positional relationships among pixels of the reference image, object image for synthesis, and synthesized image. At bottom in FIG. 1, “∘” symbols denote pixels of the reference image. “•” symbols denote pixels of the object image for synthesis. The hatched circles shown on the broken gridlines denote pixels of the synthesized image. In this drawing, resolution of the reference image and object image for synthesis are shown as being the same, with frame image resolution being increased by 1.5× in the x axis direction and y axis direction.
Here, pixel g1 of the synthesized image will be considered. This pixel g1 coincides with pixel t1 of the reference image. Here, on the basis of tone values of the four pixels s1-s4 of the object image for synthesis surrounding pixel g1, a tone value at the position of pixel g1 is calculated by a bilinear method, and this tone value is then averaged with the tone value of pixel t1 of the reference image to obtain a tone value for pixel g1.
A tone value for pixel g2 of the synthesized image is determined by the following procedure. On the basis of tone values of the four pixels t2-t5 of the reference image surrounding pixel g2, a tone value at the position of pixel g2 is calculated by a bilinear method. Next, on the basis of tone values of the four pixels s4-s7 of the object image for synthesis surrounding pixel g2, a tone value at the position of pixel g2 is calculated by a bilinear method. The two are then averaged to obtain a tone value for pixel g2.
Tone values for other pixels can be determined in the manner described above. Here, in order to facilitate understanding, the description assumes that resolution of the reference image and processing-object-image is the same; however, where reference image and processing-object-image resolutions are different, a similar process may be carried out after enlargement or reduction, as appropriate.
FIG. 2 is an illustration showing a synthesis method for use where there is zero image shift between a reference image and an object image for synthesis. At top in FIG. 2 are shown a reference image and an object image for synthesis, positioned so as to correct for shift. In this case, since shift is zero, the reference image and object image for synthesis overlap completely. At bottom in FIG. 2 are shown positional relationships among pixels of the reference image, object image for synthesis, and synthesized image. Since the reference image and object image for synthesis overlap in their entirety, the pixels of the reference image and those of the object image for synthesis are situated at the same locations.
A tone value for pixel g2 of the synthesized image is determined by the following procedure. First, on the basis of tone values of the four pixels t2-t5 of the reference image surrounding pixel g2, a tone value at the position of pixel g2 is calculated by a bilinear method. Next, on the basis of tone values of the four pixels s2-s5 of the object image for synthesis surrounding pixel g2, a tone value at the position of pixel g2 is calculated by a bilinear method. The two are then averaged to obtain a tone value for pixel g2.
Since tone values of pixels t2-t5 and tone values of pixels s2-s5 are the same values, the tone value at the position of pixel g2 calculated by bilinear method on the basis of pixels t2-t5 and the tone value at the position of pixel g2 calculated by bilinear method on the basis of pixels s2-s5 are identical values. That is, the average value thereof will also be the same as tone value at the position of pixel g2 calculated by bilinear method on the basis of pixels t2-t5, and tone value at the position of image g2 calculated by bilinear method on the basis of pixels s2-s5.
That is, where there is zero shift between a reference image and an object image for synthesis, carrying out the synthesis process will give a synthesis result that is no different from the original image. Similarly, where there is only a slight shift amount, there will be substantially no noticeable difference between the image resulting from synthesis and the original image. In such instances, the synthesis only increases the processing time, with no hope of substantial improvement in picture quality.